In recent years, the use of so-called digital wristwatches has greatly increased. These watches are characterized by the use of an electro-optic display employing light emitting diodes. A frequency standard in the form of a crystal oscillator acts through solid state electronic circuit dividers to control the light emitting diodes. A small dry cell battery is employed to energize the electrical elements.
While such wristwatches have the advantage of extreme accuracy in a digital display of the time, they have the drawback that in order to conserve the battery energy employed to energize the light emitting diodes, such watches are commonly provided with a switch which must be manipulated in some way to cause energization of the light emitting diodes. The common form of such a switch is a switch having an actuator projecting through the side of the case which must be pushed by the wearer to cause energization of the electro-optical display. The drawback to this arrangement is that it requires the use of the hand on the other arm than that on which the watch is worn, and if the wearer is in a situation where the other hand is not readily available, it becomes extremely difficult to determine what time it is. For example, if the wearer is carrying a number of packages, it is sometimes very difficult to manipulate the hand on which the watch is not being worn to a position in which this button can be closed.
As a result of this problem, various arrangements have been devised for enabling the electro-optical display to be energized by manipulation of the wrist on which the wristwatch is being worn. This usually involves an auxiliary switch which is closed by a predetermined arm movement. To guard against this auxiliary switch being turned on during normal movement of the arm on which the watch is mounted, provision is made for closing the switch only in the event of a predetermined abnormal movement off the arm. For example, one typical type of arrangement requires moving the arm between two positions within a predetermined period of time. Such arrangements are shown in the U.S. patents to Bergey Nos. 3,742,699, 3,823,550 and 3,871,170 and in the U.S. patent to Yamauchi et al., No. 3,935,701. The arrangements shown in the Bergey patents referred to also include the use of a magnetic switch which is closed by manipulation of the wrist to a position where there is a magnetic element being worn on the body such as in the belt buckle. Another arrangement proposed in at least the Bergey U.S. Pat. No. 3,742,699, is that of a diaphragm operated switch can be manipulated by movement of the arm on which the watch is being worn. Also, Bergey contemplates the use of a capacitance switch which is operated by movement of the arm to another portion of the body to change the capacitance.
All of these prior arrangements have the drawback of either increasing the complexity of the watch or of requiring a manipulation of the hand or arm which is not normal to reading of the watch. For example, those arrangements which require the movement of the wrist between two positions within a predetermined period of time require the presence of two separate switches with electronic circuitry to determine the time between the closure of the two switches and to cause energization of the electro-optical display only if the two switches are closed within a predetermined period of time. This obviously introduces additional complications into a watch which already contains some very complicated electronic circuitry. Similarly, when the display is turned on by the change in capacitance, this again requires additional circuit elements because the change in capacitance by movement of the wrist into proximity with another portion of the body is relatively small. Furthermore, all of the actions outlined above, as pointed out previously, require movements which are not normal to reading a watch.